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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Creates a new stream that repeats elements of type `A` endlessly by applying the provided closure, the repeater, `F: FnMut() -> A`."><title>repeat_with in futures::stream - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="futures" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../futures/index.html">futures</a><span class="version">0.3.28</span></h2></div><div class="sidebar-elems"><h2><a href="index.html">In futures::stream</a></h2></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../../futures/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Function <a href="../index.html">futures</a>::<wbr><a href="index.html">stream</a>::<wbr><a class="fn" href="#">repeat_with</a><button id="copy-path" title="Copy item path to clipboard"><img src="../../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../../src/futures_util/stream/repeat_with.rs.html#91">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub fn repeat_with<A, F>(repeater: F) -> <a class="struct" href="struct.RepeatWith.html" title="struct futures::stream::RepeatWith">RepeatWith</a><F><div class="where">where
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F: <a class="trait" href="https://doc.rust-lang.org/1.76.0/core/ops/function/trait.FnMut.html" title="trait core::ops::function::FnMut">FnMut</a>() -> A,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Creates a new stream that repeats elements of type <code>A</code> endlessly by
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applying the provided closure, the repeater, <code>F: FnMut() -> A</code>.</p>
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<p>The <code>repeat_with()</code> function calls the repeater over and over again.</p>
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<p>Infinite stream like <code>repeat_with()</code> are often used with adapters like
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[<code>stream.take()</code>], in order to make them finite.</p>
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<p>If the element type of the stream you need implements <a href="https://doc.rust-lang.org/1.76.0/core/clone/trait.Clone.html" title="trait core::clone::Clone"><code>Clone</code></a>, and
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it is OK to keep the source element in memory, you should instead use
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the [<code>stream.repeat()</code>] function.</p>
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<h2 id="examples"><a href="#examples">Examples</a></h2>
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<p>Basic usage:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{<span class="self">self</span>, StreamExt};
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<span class="comment">// let's assume we have some value of a type that is not `Clone`
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// or which don't want to have in memory just yet because it is expensive:
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</span><span class="attr">#[derive(PartialEq, Debug)]
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</span><span class="kw">struct </span>Expensive;
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<span class="comment">// a particular value forever:
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</span><span class="kw">let </span><span class="kw-2">mut </span>things = stream::repeat_with(|| Expensive);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Expensive), things.next().<span class="kw">await</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Expensive), things.next().<span class="kw">await</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(Expensive), things.next().<span class="kw">await</span>);</code></pre></div>
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<p>Using mutation and going finite:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::stream::{<span class="self">self</span>, StreamExt};
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<span class="comment">// From the zeroth to the third power of two:
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</span><span class="kw">let </span><span class="kw-2">mut </span>curr = <span class="number">1</span>;
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<span class="kw">let </span><span class="kw-2">mut </span>pow2 = stream::repeat_with(|| { <span class="kw">let </span>tmp = curr; curr <span class="kw-2">*</span>= <span class="number">2</span>; tmp })
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.take(<span class="number">4</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(<span class="number">1</span>), pow2.next().<span class="kw">await</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(<span class="number">2</span>), pow2.next().<span class="kw">await</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(<span class="number">4</span>), pow2.next().<span class="kw">await</span>);
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<span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(<span class="number">8</span>), pow2.next().<span class="kw">await</span>);
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<span class="comment">// ... and now we're done
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</span><span class="macro">assert_eq!</span>(<span class="prelude-val">None</span>, pow2.next().<span class="kw">await</span>);</code></pre></div>
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